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Old 08-31-2009, 10:34 PM   This thread is in the EcoModder Project Library | #1 (permalink)
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Toying with idea of taller Metro 5th gear (to go with the already taller final drive)



I'm starting this thread to pressure me a little to actually noodle this through.

What I need to figure out is whether I can even practically use a taller 5th gear, considering that my transmission swap already reduced my engine RPM in all gears for a given road speed by almost 26%. (The taller final drive lowered RPM by 19.8%. Add the tire size difference between the Swift where the transmission came from, and the Metro, and it makes for a total effecive RPM change of 25.6%.)

Why do I want an even taller 5th? Because I think it would suit my driving style for highway cruising: driving with load (DWL). I don't like using pulse & glide for long drives.

Of course, if you have a Metro and have looked into this, you already know there is no 5th gear set that can just be swapped in.

More likely it'll require a custom made set - I'm thinking something along the lines of the approach described in this thread: http://ecomodder.com/forum/showthrea...t-zx2-763.html

He took a set of 2nd gears from the same transmission, flipped and mated them to the hubs of his old 5th gear set. Dropped his RPM by nearly 1000 rpm at 70 mph, though.

I know a machinist (the guy who made the ForkenSwift's motor/transmission coupler), and he might not charge me crazy beans to do this. Certainly not the $550-800 that was quoted when someone on Teamswift offered to do custom 5th gears for people (and it never panned out).

So, I need to do the calcs. Tim (Daox) sent me a handy spreadsheet for plugging in gear ratios that spits out a nice graph of RPM / MPH. I need to actually do it with the reversed 2nd gear ratio.

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Old 09-01-2009, 12:41 AM   #2 (permalink)
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So I chose 75 km/h as an arbitrary speed at which I'd like to be able to actually use 5th gear (on mostly flat road). In other words, the RPM can't be so low that it's either lugging, or not able to produce enough power to propel the car at that speed.

A pretty picture comparing 5th gear / final drive combinations:



The ratio of the 5th gear assumes I could successfully swap the 2nd gear ratio: from 1.894 to 0.528

So, with my tall final drive, I wonder if I could drive down the road at 75 km/h at 1279 RPM. I'm not sure - have to take the car for a spin and see how it feels at 1279.
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Old 09-01-2009, 01:06 AM   #3 (permalink)
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That's approaching HF levels!!!

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Old 09-01-2009, 10:41 AM   #4 (permalink)
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Blah to new 5th gears, cooled EGR is the way of the future!
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Old 09-01-2009, 01:14 PM   #5 (permalink)
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You may try putting a billboard like device (magnetic pizza delivery sign) on the top of the car to induce more drag at your current 1279 RPM speed.

A limiting factor may be the gear ratio's available to fit the shaft spacing.
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Old 09-01-2009, 03:20 PM   #6 (permalink)
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I usually start my pulses at about 1200 rpm. That's with my 1.6L 107 hp Honda, for comparison. I would be happy with that setup, but I wonder if your smaller engine is up to the task. It's within the realm of possibility, though.

Edit: I'm finding sources that say 49 hp for the XFI, and 1830 lb, for 37 lb per hp. Compare that to my 2300 lb / 107 hp = 24.5 lb per hp. 1300 rpm might be a little low.
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Old 09-01-2009, 04:23 PM   #7 (permalink)
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Thanks for the calcs, Andrew. One thing in my favour is the car's declining aero drag as I continue testing/modding; power needed to maintain a given highway speed is dropping as a result.

I'm just about to go out and do some coastdown comparisons (front wheel skirts). While I'm out, I'll play around and see how happy the engine is loafing along in top gear at ~1300 RPM. I'm going to predict: "not very."

Quote:
Originally Posted by DonR View Post
A limiting factor may be the gear ratio's available to fit the shaft spacing.
We know that the reversing the 2nd gear set (see thread above about the Ford) will fit the shaft spacing. Whether it'll fit the stamped steel cover that houses the external 5th gear set is another question though.
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Old 09-01-2009, 04:38 PM   #8 (permalink)
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MetroMPG -

I seem to remember that when COZX2 did it to his ZX2, he had a friend machine a 5th gear set for maybe $200. But that's the insider-friend price, because COZX2 is an NHRA dude.

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Old 09-01-2009, 05:46 PM   #9 (permalink)
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Well, too much traffic to do coastdown testing.

But I did get to do a fair amount of low RPM loafing. And... 1300 RPM is doable with this engine, but barely. And if I'm saying something is barely doable for increased efficiency, then you know there's not a lot of wiggle room I wouldn't say the engine is lugging, but there is increased vibration.

Also, consider that 1300 RPM in top gear is happening at about 50 km/h with my current set up. Which takes about 2.5 horsepower to maintain. 75 km/h would take about 6 hp. (Per the default values for my unmodded car in the aero/rolling resistance tool.)

The good thing is that IF I can get a new 5th set made out of a 2nd set, I still have the original transaxle that I can put back in. That would give me the third from top gearing scenario in the chart, rather than the bottom scenario:



Anyway, I'm going to see if I can acquire an extra 2nd and 5th gear set and go visit my machinist.
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Old 09-01-2009, 05:52 PM   #10 (permalink)
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Quote:
Originally Posted by cfg83 View Post
I seem to remember that when COZX2 did it to his ZX2, he had a friend machine a 5th gear set for maybe $200. But that's the insider-friend price, because COZX2 is an NHRA dude.
I forgot that he'd had that done.

I may be lucky enough to also get some insider-friend pricing. We'll see...

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Ecodriving test:
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